1. Field of the Disclosure
The present disclosure relates generally to printers, and more particularly, to fluid ejection devices for printers.
2. Description of the Related Art
A typical fluid ejection device (heater chip) for a printer, such as an inkjet printer, includes a substrate (e.g. silicon substrate) carrying at least one fluid ejection element thereupon; a flow feature layer configured over the substrate; and a nozzle plate configured over the flow feature layer. The flow feature layer includes flow features (fluid chambers and fluid channels), and the nozzle plate includes a plurality of nozzles.
Various fluid ejection devices employ polyimide-based nozzle plates with laser ablated nozzles. In such fluid ejection devices, a fluid (such as ink) of a particular color is fed from a fluid tank to fluid ejection elements through long and large fluid through vias configured in respective substrates of the fluid ejection devices. Further, in such fluid ejection devices, comb-shaped flow feature arrays are laid out along edges of the fluid through vias, such that flow features (separating walls) of the flow feature arrays are configured perpendicularly to the fluid through vias.
As opposed to the individual assembly of the polyimide-based nozzle plates at the die level, photoimaged nozzle plate (PINP) based process proceeds in the wafer level to lithographically form fine nozzles on a laminated nozzle plate dry film. Employing PINP based process for fabricating fluid ejection devices results in benefits, such as short turnaround time, low development cost, and demonstrated consistent processing. However, the use of the PINP based process requires nozzle plates to have good photo-imageability, robust chemical properties, good thermal properties, and strong mechanical properties, which are at least comparable to that of previous polyimide-based nozzle plates.
Typically, a fluid ejection device employing a photoimaged nozzle plate, may have five fluid through vias for fluids of colors such as Cyan, Magenta, Yellow, blacK, and blacK (CMYKK). Such fluid through vias may have a dimension of about 0.2 millimeter (mm)×0.5-1 inches (″) (i.e., width×length. However, suspending a nozzle plate over such large and long fluid through vias may prove to be problematic for the processing of the nozzle plate. For example, low glass temperature (Tg) of PINP film, as used for forming the nozzle plate, may allow a narrow processing window for thermal processes, such as lamination with very tight control, post exposure bake, and final bake, needed to prevent variable large nozzle plate sagging over the fluid through vias, thereby resulting in negative effects on performance and lifetime of the fluid ejection device. Specifically, suspending the nozzle plate over the large and long fluid through vias may lead to ejected fluid droplet misdirection due to large nozzle plate sag; lamination failure while configuring the nozzle plate (particularly, above flow features and flow feature filtering pillars) because of nozzle plate elasticity change during the processing of the nozzle plate and the servicing of the fluid ejection device; fluid ingressive attack on the large exposed nozzle plate surface that may accelerate nozzle plate deformation and delamination; and so forth.
In addition, current trend of inkjet technology for achieving higher printing resolutions requires higher spatial density of nozzles with narrow flow features between firing chambers and thin nozzle plate. However, narrower flow features further weaken adhesion between the flow feature layer and either the nozzle plate or the substrate due to reduced contact area. Further, thin nozzle plate over large fluid through vias requires the nozzle plate to possess high mechanical strength and a better fluid (chemical) resistance.
Also, in a typical fluid ejection device packaging process, residual stress remains on the fluid ejection device due to mismatch of Coefficient of Thermal Expansion (CTE) between system components such as the fluid ejection devices, assembly substrate (ceramic, liquid crystal polymer or other plastics), and thermally cured adhesive, etc. For a fluid ejection device with multiple large (long) fluid through vias, each silicon section between adjacent fluid through vias responds to the residue stress differently due to non-uniform mechanical strength. Accordingly, it is difficult to maintain planarity across the fluid ejection device. Further, an uneven surface of the fluid ejection device definitely stretches the suspending nozzle plate and changes nozzle plate's surface (topography), thereby, resulting in an unpredictable factor for fluid ejection misdirection. Although the photoimaged nozzle plate is fully cured, the photoimaged nozzle plate becomes less fluid-resistant due to additional strain from the aforementioned stretching. Severe bulging of the photoimaged nozzle plate may then quickly develop above the fluid through vias leading to ejection misdirection and eventual failure of the fluid ejection device.
Till date, various attempts have been made to fabricate fluid ejection devices with shorter fluid through vias with an aim of circumventing the aforementioned problems. FIGS. 1-4 illustrate an exemplary process flow for configuring shorter fluid through vias within a substrate of a fluid ejection device. Specifically, FIGS. 1-4 depict side cross-sectional partial views of a substrate 100 (silicon substrate). As depicted in FIG. 1, the substrate 100 includes a top portion 102 and a bottom portion 104. The substrate 100 may be then etched to configure a plurality of fluid through vias 110 (short vias) within the top portion 102 of the substrate 100, as depicted in FIGS. 2 and 4. Further, the substrate 100 may be etched to configure at least one fluid flow channel, such as a fluid flow channel 120, in the bottom portion 104 of the substrate 100, as depicted in FIGS. 3 and 4. It may be evident that the fluid through vias 110 and the fluid flow channel 120 may be configured either simultaneously or in any order. Further, the fluid through vias 110 and the fluid flow channel 120 may be configured within the substrate 100 by a deep reactive ion etching (DRIE) process.
FIG. 4 depicts the substrate 100 that may be used for configuring a flow feature layer and a nozzle plate thereon for forming a fluid ejection device, such as a fluid ejection device 10, as depicted in FIG. 5. Specifically, FIG. 5 depicts a top perspective view of the fluid ejection device 10 that includes the substrate 100 with the fluid through vias 110; a flow feature layer (not shown); and a photoimaged nozzle plate 130 (hereinafter referred to as ‘PINP 130’). The PINP 130 includes a plurality of nozzles 132. The nozzles 132 may be for one or more fluid colors. In the aforementioned configuration, each firing chamber (not shown) of the fluid ejection device 10 is fed by one fluid through via of the fluid through vias 110. Thus, the spatial density of the fluid through vias 110 is the same as that of the nozzles 132. Further, the fluid through vias 110 are arranged in two rows 112, 114, as depicted in FIG. 5. Furthermore, the flow feature layer of the fluid ejection device 10 includes a plurality of flow feature channels 142 and a plurality of filtering pillars 144. Although the fluid flow channel 120 of the substrate 100 is not shown in FIG. 5, the fluid flow channel 120 is a long slot hidden underneath the rows 112, 114 of the fluid through vias 110.
Depth of a fluid through via, such as the fluid through vias 110, determines flow resistance to firing chambers, and should be uniformly small (such as about 15 microns to about 60 microns) across the fluid ejection devices, such as the fluid ejection device 10. As mentioned above, the aforementioned conventional approach (process flow of FIGS. 1-4) for such a fluid ejection device 10 employs two DRIE processes from both sides (the top portion 102 and the bottom portion 104) of the substrate 100 to define the fluid through vias 110 and the fluid flow channel 120. However, a DRIE process may only be timed to reach a depth window due to fluctuating/varying etching rate. The width of the DRIE depth window is generally associated with normal etching rate and normal etching depth. Specifically, the width of the DRIE depth window may be about 50 micrometers (μm) for an etching rate of about 25 μm/minute in a silicon wafer having a thickness of about 400-600 μm. However, an appropriate depth of a fluid through via needs to be around 30 μm, as predicted by micro fluidics to provide appropriate flow resistance. Accordingly, conventional process flows (such as the process flow of FIGS. 1-4) may be an ineffective approach for fabricating an efficient fluid ejection device.
Accordingly, there persists a need for a fluid ejection device and a method of fabricating the fluid ejection device that are capable of preventing nozzle plate sagging over fluid through vias, fluid ejection misdirection, stretching of the nozzle plate as suspended over the fluid through vias, lamination failure, fluid ingressive attack on the nozzle plate surface, bulging of the nozzle plate, and thus, failure of the fluid ejection device.